The present invention relates generally to optical communication systems, and particularly to a method for controlling dispersion compensation in optical communication systems.
Optical transmission systems, including optical fiber communication systems, have become an attractive vehicle for carrying voice and data at high speeds. However, conventional high-speed optical communications systems are prone to increased bit error rates (BER) as the bandwidth of the individual channels in the optical communications system is increased. Two such sources of this increase in error are polarization mode dispersion (PMD) and chromatic dispersion (CD). Polarization mode dispersion is a fundamental property of single mode optical fiber and components in which signal energy at a given wavelength is resolved into two orthogonal polarizations states of slightly different propagation velocity. The resulting difference in the propagation time between the polarization states is called the differential group delay (DGD), commonly symbolized xcex94xcfx84g. Chromatic dispersion (CD) results from the fact that in transmission media, such as glass optical fibers, the higher the frequency of the optical signal, the greater the refractive index. As such, higher frequency components of an optical signal will xe2x80x9cslow down,xe2x80x9d and contrastingly, lower frequency components will xe2x80x9cspeed up.xe2x80x9d Both polarization mode dispersion and chromatic dispersion may result from the ambient environment of the optical communication system. To this end, factors such as temperature, mechanical stress or strain, and impurities in the material may result in PMD and CD in the optical communication system.
In digital optical communication systems, where the optical signal is ideally a square wave, PMD and CD may cause optical pulse spreading, or pulse deformation in general. The spreading of the digital pulse in time may cause it to overflow into the time slot that has been allotted to another bit. Ultimately, the individual bits are difficult to distinguish, and inter-symbol interference (ISI) may occur. ISI may result in an increase in the BER to unacceptable levels. In order to prevent the potential problems associated with ISI, it is necessary to compensate for both CD and PMD. Moreover, in order to properly carry out the compensation for chromatic dispersion and/or polarization mode dispersion, a suitable controller for the CD compensator and/or PMD compensator is useful. Finally, in order to effectively control ISI, it is of course advantageous to compensate for both PMD and CD in the optical communication system.
Generally, compensators are specific for the type of dispersion for which compensation is being implemented. To this end, compensators that are able to compensate for the affects of polarization mode dispersion are ineffective at compensating for the affects of chromatic dispersion, and vice versa. In addition, compensation is normally carried out in a closed feedback loop, with a controller commanding the compensator(s). Conventional compensators implement a control technique in which the controller commands a compensator action that is proportional to a magnitude of the dispersion in the system as measured by the detector. Fundamentally, this type of compensation scheme is limited; this type of method is generally only accurate when the dispersion measured by the detector is proportional to the type of dispersion (PMD or CD) for which a particular compensator is effective.
In practical systems, both chromatic are polarization mode dispersion are present to some measurable degree, and most detectors cannot accurately distinguish between them. As a result, if one particular type of dispersion has been appropriately compensated by its respective compensator, there may still be dispersion in the system resulting from the other source of dispersion. This dispersion is detected by the detector and an error signal is sent to the controller. Because the command from the controller to the dispersion compensator is based on the magnitude of the dispersion, and the detector cannot distinguish the sources of dispersion, the controller will issue a control signal to the compensator to attempt to compensate for the detected dispersion. However, this particular compensator is incapable of nullifying the dispersion from the (other) dispersion source, and ultimately the dispersion compensator will introduce further dispersion into the signal. As a result of this fundamental limitation of conventional compensation schemes, the compensator may over-correct, and actually exacerbate the dispersion problem in the system. To wit, as the particular compensator attempts to correct for the dispersion caused by the other source of dispersion, the total dispersion in the system increases, and the control loop will enter into a runaway or oscillatory mode, which is undesirable and potentially unstable.
Accordingly, what is needed is a technique for reducing the ill effect of various types of dispersion, which overcomes the limitations of the conventional techniques described above.
It is therefore an object of the present invention to provide a method for controlling dispersion compensation in an optical system in a manner that decouples the control of the compensator from the bias introduced by the addition of other independent source(s) of dispersion.
In order to achieve the above and other objectives, a method for controlling a dispersion compensation comprises receiving a first measurement of dispersion at a first step; receiving a second measurement of dispersion at a second step; calculating a gradient of dispersion between the first measurement of dispersion and the second measurement of dispersion; and sending a command to a dispersion compensator based on the value of the gradient.